The present invention relates to a method for the contactless measurement of optical parameters on coating layers applied from liquid coating agents and also to a device for performing the method.
In the lacquer industry it is necessary, for example, in the production, standardization and development of lacquers to perform optical measurements, in particular colorimetric measurements, on the lacquers, for example to obtain information about whether the lacquers meet the required optical parameters, for example the required colour intensity. This may then be followed by necessary correction steps. For example, in the production of pigmented coating agents, shading steps are necessary to establish the required shade precisely. After every shading step, the lacquer has to be lacquered onto a metal test sheet and dried or cured. The "actual-state metal sheet" obtained in this way can then be compared with a "required-state metal sheet". In the case of plain colouring, it may be necessary, in addition, to produce initially a white-lacquer mixture of every shade. In the standardization of pigment pastes, this is additionally supplemented, furthermore, by lacquering with suitable binders prior to the application as an additional working step. The work, time and cost expenditure is appreciable.
In principle, optical measurements may be made not only on applied and dried or cured lacquer layers, but also on wet lacquers if the correlation between optical measurement results on wet lacquer and on the applied and dried or cured lacquer layer is known. Whenever possible, there is therefore the desire to perform optical measurements on wet lacquers and colorimetric methods have been developed for wet lacquers. For example, measuring cells are known for wet-lacquer measurement. These are, for example, glass measuring cuvettes that can be placed in front of the measurement aperture of a colorimeter. The disadvantage of these simple measuring cells is that the accuracy of the measurement results achieved with them is unsatisfactory, for example due to rapid sedimentation of the lacquer in the cuvette or to the influence of the glass as an intervening medium, for example as a result of physical interactions. The method is completely unsuitable for colorimetric measurements on special effect lacquers.
DE-A-25 25 701 describes a method of colorimetric measurements on liquid lacquers. In this method, a continuous film of the lacquer to be investigated is formed and a portion of said film is measured colorimetrically. In this method, the lacquer film may be a film that moves with a substrate or a film moving past over a substrate with laminar flow. In the first case, the substrate is a measuring disc that can be rotated around a horizontal axis and, in the second case, it is a plate-like body having an approximately perpendicular surface which the lacquer runs down and drips off at the bottom. In both cases, the lacquer is applied by means of a casting tank having an outlet slit to the particular substrate. A disadvantage of said method if a rotatable measuring disc is used is that it is only possible to work with an acceptable disc size if colorimeters are used that have a measurement aperture disposed laterally on the housing. In the case of colorimeters having centrally disposed measurement apertures, extremely large discs have to be employed. A disadvantage of colorimetric measurements is that the measurement result, in particular in the case of special effect lacquers, vary for one and the same wet-lacquer sample depending on the point of measurement on the rotating disc. If the plate-type body having an approximately vertical surface is used, it is only possible to work with wet-lacquer sheet thicknesses above the sagging limit. In particular, colorimetric measurement can consequently not be meaningfully performed on special effect lacquers.
DE 36 38 932 C2 describes an on-line method of continuously determining the coating amount of silicones on a continuous paper or plastic web to be coated. In this case, the gloss before and after the coating is determined with two separate measuring units and the amount applied is determined therefrom.